In general, Bistable Liquid Crystal Displays, and in particular, Cholesteric Liquid Crystal Displays (ChLCDs) (employing what is also called chiral nematic liquid crystals), have proven to have great potential to create low cost pressure sensitive eWriters that are efficient power consumers and that can be utilized in a number of unique devices.
Recently, the BOOGIE BOARD® pressure sensitive cholesteric liquid crystal eWriter of Kent Displays Inc. has appeared on the market in which a pointed stylus or the finger can be used to write or trace an image on the surface of the eWriter device as described in U.S. Pat. No. 6,104,448, which is incorporated herein by reference in its entirety. As used in this disclosure, the term eWriter includes any device including substrates and an electrooptical material disposed between the substrates, which exhibits a change in reflectance when writing pressure is applied to one of the substrates, the device being able to be erased upon application of a voltage. This definition of an eWriter excludes devices that are only able to be written on by digitally electrically addressing the liquid crystal as in the case of a laptop that employs a display actively driven with a thin film transistor (TFT) array. The cholesteric liquid crystal eWriter offers a considerable improvement over previous technologies in that the image can be simply and instantly erased with the push of a button that applies a voltage pulse to electrically conductive electrodes in the eWriter.
Both substrates of the cholesteric liquid crystal eWriter are flexible and the bottom substrate is painted with an opaque light absorbing dark background. The upper substrate and upper electrode are optically clear and include electrically conductive layers on their interior surfaces spaced apart by a cell gap. Within the cell gap is a bistable cholesteric liquid crystal which can exhibit two textures, a substantially transparent (focal conic) texture and a color reflective (planar) texture. The spacing of the cell gap is usually set by plastic or glass spacers that are either cylindrical or spherical in shape. The cholesteric liquid crystal eWriter is initialized by applying voltage pulses to the electrodes to electrically drive the cholesteric material to the substantially transparent texture. When one presses on the top substrate with a pointed stylus or finger, the liquid crystal is locally displaced. Flow induced in the liquid crystal changes its optical texture from substantially transparent to a reflective color at the location of the stylus. The reflective color contrasts well with the dark background of the lower substrate. An image traced by the stylus or finger nail will remain on the eWriter device indefinitely without application of a voltage until erased. Erasure is accomplished by applying a voltage pulse to the transparent conducting electrodes on the interior surfaces of the two substrates that drive the cholesteric liquid crystal from its color reflective state back to its substantially transparent state.
The image written on an eWriter is viewable optically but can't be saved utilizing just the eWriter. A digitizer is required either behind or in front of the eWriter for capturing images. This is previously demonstrated in Patent Application Publication 2010/0265214. However, numerous types of digitizers are currently on the market, those that allow for saving from finger or gesture input and those that allow for stylus input. The eWriter device requires a digitizer that utilizes stylus input. Another distinction between different types of digitizers is that the digitizer can either allow input from any stylus or will require input from a dedicated stylus as shown by the commercially available Sync™ eWriter, as disclosed in Patent Application 2014/0043547, which employs an Electromagnetic resonance (EMR) digitizer behind the eWriter. There are many issues with a dedicated stylus including: cost—since miniaturized electronics are required inside the stylus, ease of use—users can only use one stylus that once lost makes the device unusable, and user confusion—the user can still achieve a visually written image on the eWriter when using a different stylus even though the digitizer will not record the written image when the dedicated stylus is not used.
Lower cost digitizers tend to include those of the resistive type. Resistive digitizers have the advantage that they are pressure sensitive and function without the need of a dedicated stylus. Resistive digitizers can be 4 wire, 5 wire, 8 wire, among others. These resistive digitizers can utilize any stylus and do not require a dedicated stylus. Unfortunately, some resistive digitizers do not have palm rejection, meaning if a user puts their palm down while writing the palm will result in a data point. Since these resistive digitizers do not allow for multiple data points the data will be corrupted and result in scribbles across the screen. Therefore, consumers would benefit from an eWriter that can employ a digitizer that allows for stylus writing with palm rejection, that can use any stylus, even a fingernail, and is also low cost.
Other products on the market, such as tablets, use a different type of digitizer called Projected Capacitive (ProCap), where a finely patterned conductive film or films is placed over the top of the display. The ProCap type digitizer works by measuring a capacitance change on the grid of finely patterned conductors when a finger or other item of sufficiently large capacitance comes close to the digitizer. For ProCap digitizers it is possible to have palm rejection programmed into the software. It is not possible with a 4 wire, 5 wire, or 8 wire resistive type digitizer to have palm rejection in the software as multiple touches that register electronically result in corrupted data. Therefore, devices with a ProCap type digitizer are not considered to be relevant to this disclosure.
A problem with the prior art is that the eWriter alone does not digitally save information. If information written on the eWriter needs to be saved, a separate digitizer can be attached to the eWriter to sense and save writing while the eWriter visually displays the writing. In the prior art a separate electromagnetic resonance (EMR) type digitizer is placed below the eWriter, namely, the Boogie Board® Sync™eWriter system. This system has several issues in that it is somewhat expensive and requires a dedicated stylus, meaning only the provided stylus with electronics creates a saved file from the written image. The user could accidentally use a simple plastic stylus and create a visible written note on the Sync™ eWriter system. However, there would not be an electronic image to save due to the wrong stylus being used. Since the sensing and optical response are decoupled, the user does not get the optical feedback seen on a typical display to indicate whether one is using the correct stylus.
Previously designed resistive digitizers have not been commercially used with an eWriter having mechanical palm rejection under ordinary writing conditions. It is not possible to place a typical resistive digitizer behind an eWriter to produce a working system that has both palm rejection and allows for capture of light stylus writing. The typical resistive digitizers would not allow for palm rejection if they would be placed under the eWriter. Since the sensing and optical response are decoupled in a cholesteric eWriter, as previously mentioned, the user would not be able to see in real time what amount of writing pressure needs to be applied to create a defect free saved written image. Consumers would greatly benefit from a low cost eWriter system that has a resistive digitizer and provides palm rejection under ordinary writing conditions, reads lightly written strokes, and can be used with any stylus or even a fingernail.
Currently, commercially available Boogie Board® Sync™ eWriter systems employ an EMR type digitizer, as shown in PRIOR ART FIG. 1. This configuration includes a Typical eWriter 10 that includes a top substrate 11 and a bottom substrate 12. Each substrate has electrodes 14 on the interior surface that are used to electrically drive the Typical eWriter and erase images written on the eWriter. The upper substrate 11 can have additional functional coatings such as anti-glare, anti-fingerprint, hard coat and others. The lower substrate 12 typically has a light absorbing coating 15 that allows for contrast in the written eWriter and prevents visibility of the EMR digitizer underneath the eWriter. Between the electrodes 14 is disposed a dispersion layer 13 including cholesteric liquid crystal (ChLC) and polymer with spacers that allow for images to be written on the eWriter. The top substrate 11 and bottom substrate 12 and their corresponding electrodes 14 go out to eWriter ledges that connect to electronics to switch the eWriter. Under the eWriter is an EMR digitizer 40 that has electrical leads 44 that transmit electrical signals indicating the location of the EMR stylus 20. Between the Typical eWriter 10 and the EMR digitizer is a plastic sheet 46 that is used in the housing of the device. The EMR stylus is relatively complex and costly as it includes electronics on a printed circuit board (PCB) 22. As the EMR stylus 20 is pressed into the eWriter 10 the upper substrate 11 depresses 18 and allows for a bright written line to appear on the eWriter. In addition to writing on the eWriter, the upper substrate 11 depression 18 triggers a spring in the tip 24 of the EMR stylus 20 causing written data to be recorded by the EMR digitizer 40. Only when using the EMR stylus 20 will writing/drawing information be tracked and recorded by the eWriter/EMR digitizer system. The Typical eWriter itself will respond to any written pressure from any simple plastic stylus or even a fingernail. All that is required to make an image is to apply localized pressure to the eWriter. Therefore, with the Typical eWriter/EMR system users have the possibility of seeing a written image on the Typical eWriter when a non-EMR stylus is used, but in that case no data is recorded by the EMR digitizer and the image is not saved.